Food hygiene regulations impose increasingly strict requirements for the handling and storage of foodstuffs, and it is desirable to be able to establish reliably that, over a given period, the correct storage temperature has been maintained to suppress the growth of bacteria, for example. Various devices are available which can be placed in the food container, for example a vehicle, to accompany the food and to measure and record the temperature within the container at predetermined intervals, recording at the same time the date and time of the measurement. When the food reaches its destination, the temperature and time measurements recorded in the device are read into a computer by connecting the device to a reader unit.
In order to ensure sufficient battery power is available to operate data measuring and recording functions reliably over a long period, and to permit transfer of the data to the reader unit, known devices generally are relatively bulky and costly to manufacture. This limits their application, since they occupy space which would otherwise be occupied by foodstuffs, and the cost of including one or more of the devices with every load of foodstuffs can be prohibitive. As a result, therefore, most existing devices tend to be used for random testing, travelling with selected loads, rather than for monitoring every load and thus enabling responsibility for spoiled foods to be established, or at least to be avoided, by the user of the devices. There is therefore a need for a temperature monitoring system which includes small and relatively inexpensive temperature monitoring devices which can travel with every vehicle load, or even with every pallet load within the vehicle.
The specification of U.S. Pat. No. 4,972,099 discloses a portable sensor card in which an external physical phenomenon can be sensed and the sensed value subjected to signal and data processing and the results of processing stored, and in which the stored data can be read out to an external device. More particularly this publication teaches a sensor card comprising a single card substrate on which are mounted a sensor, an integrated circuit having a memory, and an output terminal. The sensor card is used to detect and record the temperature of the environment of food or living things during transportation and includes a sound-emitting element for generating a predetermined warning tone when the microprocessor detects a sensed temperature outside allowable limits. The sensor card incorporates its own power supply, in the form of a battery imbedded within the card substrate, and has an external connection terminal for connection to an external device such as a personal computer. This sensor card is relatively bulky due to the imbedded battery which needs to meet all its energy requirements. Furthermore, the external connection terminal and the projecting sound-emitting element both provide recesses in which foodstuffs, dirt and associated micro-organisms will accumulate to the detriment of future consignments.